Method and machine for producing multiaxial fibrous webs

ABSTRACT

Method and machine for producing multiaxial fibrous webs. A plurality of unidirectional sheets ( 30   a,    30   b,    30   c ) are superposed in different directions and they are bonded together. At least one of the unidirectional sheets is made by spreading a tow so as to obtain uniform thickness, width not less than 5 cm, and a weight of no more than 300 g/m 2 , cohesion being imparted to the sheet so as to enable it to be handled prior to being superposed with other sheets. Advantageously, the unidirectional sheets are made of carbon fibers and are obtained by spreading out large tows.

FIELD OF THE INVENTION

[0001] The invention relates to making fiber sheets, and moreparticularly multiaxial sheets formed by superposing and linkingtogether a plurality of unidirectional fiber sheets disposed indifferent directions.

[0002] A field of application of the invention lies in making multiaxialfiber sheets for forming reinforcing plies for preparing compositematerial parts. The intended materials are particularly thoseconstituted by fiber reinforcement which can be organic or inorganic, orprecursors therefor such as fibers of polymer, glass, carbon, ceramic,para-aramid, . . . , which reinforcement is densified by an organicmatrix, e.g. a resin, or an inorganic matrix, e.g. glass, carbon, orceramic.

STATE OF THE ART

[0003] It has been known for a long time to make multiaxial fiber sheetsby superposing unidirectional sheets, i.e. made up of threads or fibersthat are oriented essentially in a single direction, the unidirectionalsheets being superposed in different directions.

[0004] A common technique consists initially in making theunidirectional fiber sheets, and in giving them sufficient cohesion toenable them to be handled without dispersing the elements making themup.

[0005] A commonly proposed solution consists in bonding together theelements forming the warp of the unidirectional sheets by threadsextending in the weft direction. This inevitably results in undulationsbeing formed which, when a plurality of sheets are superposed andpressed against one another, can cause fibers to be crushed and broken,thereby creating discontinuities. That degrades the multiaxial sheetsmade in that way and consequently degrades the mechanical properties ofthe composite material parts prepared from such multiaxial sheets.

[0006] To remedy that drawback, a well-known solution consists in usingbonding threads of number and weight that are as small as possible.Document GB-A-1 190 214 (Rolls Royce Limited) concerning sheets ofcarbon precursor fibers, and document FR-A-1 469 065 (Les Fils d'AugusteChomarat & Cie), concerning sheets of glass fibers, illustrate thatapproach. Nevertheless, it is clear that the above-mentioned drawback isdiminished but not eliminated.

[0007] It is also proposed in document EP-A-0 193 478 (EtablissementsLes Fils d'Auguste Chomarat & Cie) to use bonding fibers but made of aheat-fusible material. During the preparation of the composite material,the temperatures used can cause the bonding threads to melt at least inpart, thereby reducing the extra thickness where they cross the warpelements. However it is necessary for the material of the bonding fibersto be compatible with the nature of the matrix of the compositematerial, which greatly limits the use of that method.

[0008] Another solution mentioned in document FR-A-1 394 271 (Les Filsd'Auguste Chomarat & Cie) consists in placing glass fiber threadsparallel to one another and in bonding them together chemically, thebinder used being soluble in the matrix. In that case also, the need forcompatibility between the binder and the matrix limits applications ofthe method. Furthermore, no means is described to enable the threads tobe placed parallel to one another, and it will readily be understoodthat making wide sheets on an industrial scale gives rise to realpractical difficulties. Finally, the resulting sheet is not free fromundulations resulting from the threads being placed side by side.

[0009] Yet another solution consists in spreading out a plurality oftow, bringing together the resulting unidirectional fiber strips in aside by side configuration to form a sheet, and in imparting transversecohesion to the sheet by needling. Such a method is described inparticular in document U.S. Pat. No. 5,184,387 (assigned to AerospacePreforms Limited) where the tows used are made of carbon precursorfibers capable of being needled without being broken. Nevertheless,multiaxial sheets are not made by superposing those unidirectionalsheets. According to that document, annular sectors are cut out from theunidirectional sheet to form annular plies which are superposed andneedled.

[0010] To avoid the need to give even temporary cohesion tounidirectional sheets for making multiaxial sheets, it is known to makethe multiaxial sheets directly by forming a plurality of unidirectionalsheets and by superposing them in different directions without anyintermediate handling. The superposed sheets can be connected to oneanother by bonding, by sewing, or by knitting.

[0011] Documents illustrating that technique are, for example,documents: U.S. Pat. No. 4,518,640, U.S. Pat. No. 4,484,459, and U.S.Pat. No. 4,677,831.

[0012] In document U.S. Pat. No. 4,518,640 (assigned to Karl Mayer)reinforcing threads are introduced into the sheet while it is beingformed, thereby making it possible to provide bonding without piercingthrough the fibers. Nevertheless, that gives rise to openings beingpresent in the multiaxial sheet, which openings produce surfacediscontinuities.

[0013] In document U.S. Pat. No. 4,484,459 (assigned to Kyntex Preform),each unidirectional sheet is formed by causing a thread to pass aroundspikes carried by two parallel endless chains, such that the portions ofthe threads that extend freely between the spikes are mutually parallel.Unidirectional sheets are formed by guiding the respective threads indifferent directions, and they are bonded to one another by sewing. Withthat technique it is not possible to have reinforcing threads in thelongitudinal direction of the multiaxial sheet; unfortunately, it isoften necessary to place reinforcing elements in that main direction. Inaddition, if a large amount of tension is exerted on the threads toguarantee parallelism in each sheet, then the portions of the threadsextending between the spiked chains can tend to become rounded by thefibers tightening, thereby giving rise to openings in the multiaxialsheet. Finally, it will be observed that that technique does not make avery high production speed possible given the time required for formingeach unidirectional sheet.

[0014] In document U.S. Pat. No. 4,677,831 (assigned to LibaMaschinenfabrik GmbH), the technique described consists in displacing amain unidirectional sheet longitudinally parallel to the direction ofthe elements which make it up, and in laying transverse unidirectionalsheets thereon in directions that make predetermined angles with thedirection of the main sheet (0°), for example +45° and −45° and/or +60°and −60°. The transverse sheets are laid by a laying process between twospiked chains situated on either side of the main sheet. That techniquewhich does not necessarily require a main sheet to be present, alsosuffers from several drawbacks.

[0015] Thus, it is necessary to eliminate the marginal zones where thetransverse sheets turn around the spikes. Unfortunately, the wider thetransverse sheets, the larger the marginal zones, and the larger thelosses of material due to their being eliminated, and it is also moredifficult to turn the sheets on the spikes. This greatly limits thewidth that can be used for the transverse sheets. In addition, theabove-mentioned drawback of possible irregularity in the multiaxialsheet is also to be found, in particular due to the formation of holesbecause of the tensions that it is necessary to apply to the elements ofthe transverse sheets in order to hold them parallel during laying.

[0016] In addition, relatively high stitch density is necessaryimmediately after laying in order to confer sufficient strength to theresulting multiaxial sheet. In addition to making it impossible topreserve a smooth surface state, this high stitch density affects theflexibility of the multiaxial sheet and limits its deformability in use,e.g. by draping.

[0017] Furthermore, when a main sheet (0°) is provided, it is necessaryto support it while the transverse sheets are being laid, such that allof them are to be found on the same side of the main sheet. Reinforcingelements are indeed provided that extend in the main direction (0°), butthe resulting multiaxial sheet is not symmetrical between its faces.Unfortunately, such symmetry is advantageous to facilitate theconstruction of regular reinforcement and it is therefore desirable toplace the main direction at 0° in the middle of the multiaxial sheet,between its faces.

[0018] It should also be observed that a drawback common to thosetechniques using threads for forming unidirectional sheets lies inobtaining multiaxial sheets which firstly present surface roughness dueto the threads, and secondly cannot be as thin as it is sometimesdesired.

[0019] Finally, a method of making a multiaxial sheet fromunidirectional sheets is also described in document GB-A-1 447 030(Hyfil Limited). A first unidirectional sheet of warp-forming carbonfibers is pre-needled and another, weft-forming unidirectional sheet isbonded to the first, likewise by needling. The pre-needling of the firstsheet seeks to displace fibers from the side where the second sheet isto be placed, in order to contribute to bonding therewith. It will beobserved that the unidirectional sheets used are made coherent by abonding thread, as described in above-mentioned document GB-A-1 190 214,with the drawbacks that result therefrom.

[0020] It should also be observed that the above-mentioned knowntechniques all suffer from a drawback which lies in the relatively highcost of multiaxial fiber sheets when they are made using carbon fibers.There exists a need to reduce the cost of such sheets, in particular soas to extend their field of application.

OBJECTS OF THE INVENTION

[0021] An object of the invention is to propose a novel method of makingmultiaxial fiber sheets, in particular to enable the cost of making suchsheets to be reduced, so as to cause multiaxial sheets made with fibersthat have the reputation of being expensive, such as carbon fibers, tobe more attractive.

[0022] Another object of the invention is to propose a method enabling“mirror” multiaxial sheets to be made, i.e. multiaxial sheets presentingsymmetry relative to a midplane, in particular relative to a mainunidirectional sheet (0°), which sheet is therefore situated betweentransverse unidirectional sheets making opposite angles relative to themain direction.

[0023] Another object of the invention is to propose a method enablingmultiaxial fiber sheets to be made that present a surface of smoothappearance without irregularities such as holes or roughnesses.

[0024] Another object of the invention is to propose a method enablingmultiaxial fiber sheets to be made requiring only a very low density ofbonding transversely to the unidirectional sheets making them up inorder to ensure coherence, thereby enabling good deformability of themultiaxial sheets to be preserved.

[0025] Another object of the invention is to provide multiaxial fibersheets having the above properties while also being of great length, andof small thickness and weight (per unit area).

[0026] Another object of the invention is to propose a laying method andmachine enabling multiaxial fiber sheets to be made from unidirectionalsheets that can be relatively wide, while conserving good surfaceregularity and limiting losses of material.

BRIEF SUMMARY OF THE INVENTION

[0027] In one aspect, the invention provides a method of making amultiaxial fiber sheet, the method comprising the steps consisting insuperposing a plurality of unidirectional sheets in differentdirections, and in bonding the superposed sheets together,

[0028] in which method, to make at least one unidirectional sheet, atleast one tow is spread so as to obtain a sheet of substantially uniformthickness, having a width of not less than 5 cm and a weight of not morethan 300 grams per square meter (g/m²), and cohesion is imparted to theunidirectional sheet enabling it to be handled prior to being superposedwith at least one other unidirectional sheet.

[0029] In a feature of the method, to make at least one of theunidirectional sheets, a plurality of tows are used, the tows are spreadso as to form unidirectional strips, and the strips are placed side byside so as to obtain a unidirectional sheet having a width of not lessthan 5 cm and weighing not more than 300 g/m².

[0030] To further improve an advantage of the method, in particular whenusing carbon, at least one of the unidirectional sheets is preferablyobtained by spreading at least one tow having a number of filamentsequal to or greater than 12 K (12,000 filaments) and possibly as many as480 K (480,000 filaments) or more.

[0031] A similar technique can be used with all technical fibers.

[0032] An advantage of the method is thus to use large tows, inparticular the largest tows available for various kinds of fiber.

[0033] For given weight, particularly with carbon, the cost of a fat towis much less than that of a thin tow or thread of the kind which, so faras the Applicants are aware, are those used in the state of the art formaking multiaxial sheets.

[0034] By way of illustration, the following table applies tocommercially available carbon threads or tows formed using differentnumbers of filaments, and gives the weights that can be obtained for aunidirectional sheet, depending on whether it is formed by mutuallyparallel threads as in the prior art, or by spreading tows as in thepresent invention. The threads or tows are made of high strength or highmodulus carbon with a polyacrylonitrile or an anisotropic pitchprecursor. Weight Unidirectional Thread or tow Unidirectional sheet madeby Number of sheet made up of spreading and filaments parallel threadsfixing  3 K 150 to 200 g/m²  6 K 200 to 250 g/m²  12 K 250 to 300 g/m²100 to 150 g/m²  50 K 100 to 250 g/m² 320 K 100 to 300 g/m² 480 K 200 to300 g/m²

[0035] A tow is spread or a plurality of tows are spread and juxtaposed,so as to form at least one unidirectional sheet having weight per unitarea no greater than 300 grams per square meter (g/m²), thus making itpossible from a limited number of heavy tows to provide a sheet ofrelatively broad width, i.e. at least 5 cm, and preferably at least 10cm.

[0036] The use of unidirectional sheets of relatively light weight makesit possible to conserve this property in multiaxial sheets made up ofsuch unidirectional sheets.

[0037] In addition, contrary to the above-mentioned prior art techniquesusing sheets of parallel threads, spreading tows until lightweightsheets are obtained causes multiaxial sheets to be made that do not havesurface defects such as holes or undulations, and that have smoothsurface appearance. It is also possible with the method of the inventionto use fibers that are fragile.

[0038] When the unidirectional sheet is built up from discontinuousfilaments, cohesion can be imparted thereto by matting the filaments toa small extent. To this end, the sheet can be subjected to needling orit can be exposed to a jet of water under pressure, the sheet beingdisposed over a plate. The sheet can then be widened without losing itscohesion.

[0039] In all cases, regardless of whether the unidirectional sheet ismade of filaments that are continuous or discontinuous, cohesion can beimparted thereto by providing a chemical bonding agent which mayoptionally be suitable for being eliminated (or sacrificed). The agentis advantageously applied by spraying a liquid compound onto the sheetor by passing it through a bath. Cohesion can also be provided bydusting a heat-fusible or thermo-adhesive polymer in powder form ontothe sheet.

[0040] It is also possible to envisage imparting transverse cohesion toat least one of the unidirectional sheets used by fixing by means of atleast one heat-fusible or thermo-adhesive film or thread, or indeed byforming a line of adhesive, e.g. an adhesive in solution in anevaporatable solvent.

[0041] The method of the invention seeks more particularly to make acontinuous multiaxial sheet having a longitudinal direction, by fetchingat least one transverse unidirectional sheet onto a moving support thatmoves in a direction of advance parallel to the longitudinal directionof the multiaxial sheet, the or each transverse unidirectional sheetbeing fetched as successive segments that are adjacent or that overlapin part and that form the same selected angle relative to the directionof advance.

[0042] The cohesion of the superposed unidirectional sheets makes itpossible to make multiaxial sheets without constraints on laying theunidirectional sheets relative to one another, thus providing greatflexibility concerning the order in which the unidirectional sheets aresuperposed. It is thus possible to make multiaxial sheets that presentsymmetry relative to a midplane (“mirror” symmetry), in particularrelative to a longitudinal middle unidirectional sheet whose directionis parallel to the direction of advance, together with at least twotransverse unidirectional sheets disposed on either side of thelongitudinal sheet and forming opposite angles relative thereto.

[0043] In a preferred implementation of the method, each of thesuccessive segments forming a transverse sheet is fetched by moving thesheet over a length substantially equal to the dimension of themultiaxial sheet as measured parallel to the direction of the transversesheet, by cutting off the segment fetched in this way, and by depositingthe cutoff segment on the moving support or the multiaxial sheet that isbeing made. Advantageously, the transverse sheet is reinforced in thezones where it is cut, e.g. by fixing a film on at least one of itsfaces.

[0044] It will be observed that laying transverse sheets in successivecutout segments makes it possible to limit losses of material comparedwith the known technique of laying by turning the sheet around spikes.In addition, working in this way avoids damaging the fibers, andtherefore makes it possible to lay fibers that are fragile, such as highmodulus carbon fibers or carbon fibers based on anisotropic pitch, orceramic fibers. In addition, restarting the laying process after a breakin transverse sheet feed is made much easier compared with the casewhere the transverse sheets are formed by a set of parallel fibers thatare not bonded together.

[0045] In another aspect, the invention provides a unidirectional ormultiaxial fiber sheet as obtained by the above method.

[0046] In yet another aspect, the invention provides making compositematerial parts that comprise fiber reinforcement densified by a matrix,in which parts the fiber reinforcement is made from at least one suchunidirectional or multiaxial sheet.

[0047] In a further aspect, the invention provides a laying machineenabling the preferred implementation of the method to be performed.

[0048] To this end, the invention provides a laying machine for making amultiaxial fiber sheet by superposing unidirectional fiber sheets indifferent directions, the machine comprising:

[0049] apparatus for advancing the multiaxial sheet, the apparatuscomprising support means for supporting the multiaxial sheet that isbeing made and drive means for driving the support means in a directionof advance;

[0050] feed means for feeding longitudinal unidirectional sheet in adirection parallel to the direction of advance;

[0051] a plurality of cross-laying devices each including feed means forfeeding the cross-laying device with continuous unidirectional sheet, amoving grasping head for taking hold of the free end of a sheet, andmeans for laying successive segments of sheet parallel to a transversedirection at a selected angle relative to the direction of advance, saidlaying means comprising means for driving the grasping head; and

[0052] bonding means for bonding the superposed unidirectional sheetstogether, the bonding means being located downstream from the supportmeans in the direction of advance,

[0053] in which machine:

[0054] each cross-laying device includes cutter means; and means areprovided for performing successive cycles comprising, for eachcross-laying device, grasping the free end of a unidirectional sheet bymeans of the grasping head, moving the grasping head to fetch a segmentof unidirectional sheet, cutting off the fetched segment ofunidirectional sheet, and laying the cutoff segment of unidirectionalsheet on the support means.

[0055] An important advantage of such a machine lies in the possibilityof laying unidirectional sheets of relatively broad width, including inthe transverse directions.

[0056] Superposed unidirectional sheets can be bonded together invarious ways, e.g. by sewing, by knitting, by needling, or by adhesive,e.g. by spraying an adhesive agent or by inserting a heat-fusible orthermo-adhesive film or thread between the sheets. A bonding agent thatmay possibly have been used for providing cohesion within unidirectionalsheets can be reactivated to bond the sheets to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The invention will be better understood on reading the followingdescription given by way of non-limiting indication with reference tothe accompanying drawings, in which:

[0058]FIG. 1 is a fragmentary overall view of an installation enablingcoherent unidirectional sheets to be made;

[0059]FIG. 2 is a fragmentary diagrammatic plan view of the FIG. 1installation;

[0060]FIG. 3 is a fragmentary view of a first variant embodiment of thecohesion means of the FIG. 1 installation;

[0061]FIG. 4 is a fragmentary view of a second variant embodiment of thecohesion means of the FIG. 1 installation;

[0062]FIG. 5 is a diagrammatic view showing part of the making andwidening of a coherent unidirectional sheet that is made up ofdiscontinuous fibers;

[0063]FIGS. 6A and 6B are a highly diagrammatic overall plan view of alaying machine for making multiaxial fiber sheets in an implementationof the invention;

[0064]FIG. 7 is a diagrammatic elevation view showing a detail of thedevice for putting local reinforcing films into place in the machine ofFIGS. 6A-6B;

[0065]FIGS. 8A to 8C show the successive steps of putting thereinforcing film into place using the FIG. 7 device;

[0066]FIG. 9 is a diagrammatic view in lateral elevation showing adetail of the device in the machine of FIGS. 6A-6B for cutting thetransverse unidirectional sheet into segments and for fixing a cutoffsegment;

[0067]FIG. 10 is a diagrammatic end elevation view of the cutting andfixing device of FIG. 9;

[0068]FIGS. 11A to 11C show the successive steps of fetching, cutting,and fixing a segment of transverse unidirectional sheet in the machineof FIGS. 6A-6B;

[0069]FIG. 12 is highly diagrammatic and shows part of a variantembodiment of the laying machine of FIGS. 6A-6B;

[0070]FIGS. 13A to 13D show the successive steps of fetching, cutting,and fixing a segment of a transverse unidirectional sheet in anothervariant embodiment of the laying machine of FIGS. 6A to 6B;

[0071]FIG. 14 is highly diagrammatic and shows a variant implementationof the fixing of segments of transverse unidirectional sheet in a layingmachine such as that of FIGS. 6A-6B;

[0072]FIG. 15 is highly diagrammatic and shows a variant implementationof laying transverse unidirectional sheets;

[0073]FIG. 16 is highly diagrammatic and shows a variant implementationof laying in which the transverse unidirectional sheets overlappartially; and

[0074]FIGS. 17, 18, and 19 are highly diagrammatic and show first,second, and third variant embodiments of the means for bonding togetherthe superposed unidirectional sheets in a laying machine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0075] Making a Unidirectional Sheet (FIGS. 1 to 5)

[0076] Tows are spread individually and the resulting unidirectionalstrips are optionally juxtaposed to form a unidirectional sheet whosecohesion is provided by supplying a bonding or attaching agent betweenthe filaments making up the sheet, prior to storing the sheet on a reel.

[0077] In FIG. 1, a single tow spreading device is shown so as toclarify the drawing. A tow 10 a is taken directly from a box in which itwas stowed. In a variant, the tow can be taken from a reel carried by acreel.

[0078] Tows of various kinds can be used depending on the use intendedfor the sheet. For example, the tows may be of carbon fibers or ceramicfibers, or of fibers that are precursors of carbon or ceramic, glassfibers, aramid fibers, or a mixture of different kinds of fiber.Suitable ceramics are in particular silicon carbide and refractoryoxides, e.g. alumina and zirconia. The tows can be made of continuousfilaments or of discontinuous filaments, and if they are discontinuousthey can be obtained, for example, by bursting tows of continuousfilaments. With tows made of discontinuous filaments, it is possible touse hybrid tows comprising filaments of different materials that areintimately mixed together. That can be achieved by fetching burst towsor ribbons made of different materials and mixing the fibers thereof bypassing them through a gill box.

[0079] When possible, heavy tows are used, specifically for the purposeof reducing the cost price of the resulting sheets. The term “heavy” towis used herein for a tow made up of at least 12 K filaments (i.e. a towmade up of 12,000 filaments), preferably a tow having a number offilaments not less than 50 K, and possibly as many as 480 K or evenmore.

[0080] The tow 10 a passes over a picker and disentangler device 12formed by a plurality of bars 12 a extending between two end plates 12b, the entire assembly being rotated about an axis parallel to the barsunder drive from a motor 13. The bars 12 a, e.g. four bars, are disposedregularly around the axis of rotation.

[0081] After passing over two deflector rolls 14 and 16 mounted torotate freely, the tow 10 a reaches a tension-adjustment device 18 madeup of four rolls 18 a, 18 b, 18 c, and 18 d that are likewise mounted torotate freely. These rolls constitute, in well-known manner, aparallelogram that is deformable under drive from an actuator 19 whichmakes it possible by acting on arms carrying the rolls to lengthen orshorten the path of the tow 10 a through the tension-adjusting device soas to keep the tension constant.

[0082] Thereafter, the tow 10 a passes successively over a plurality offixed curved rolls 22 a, 22 b, 22 c that are “banana” shaped. Theserolls, of which there may be three for example, operate in known mannerto spread out the ribbon so as to form a thin unidirectional strip 20 a.

[0083] The tension in the strip 20 a is measured in conventional mannerby passing over rolls 24 a, 24 b, and 24 c, in which the roll 24 b ismovable vertically while being biased by an elastic force. Informationabout variation in the tension of the strip as supplied by measuring thedisplacement of the axis of the roll 24 b is used to control theactuator 19 so as to keep the measured tension constant.

[0084] The strip 20 a is placed adjacent to other strips 20 b, 20 c, 20d, and 20 e that are identical or similar on a roll 25 that is free torotate, thereby forming a unidirectional sheet 30. The strips can thuscome from tows that are identical or different, e.g. if different, fromtows of different weights, or made of fibers of different kinds, therebymaking it possible to obtain a hybrid sheet.

[0085] The strips 20 b to 20 e are obtained by means of tow-spreadingdevices identical to the device described above.

[0086] As shown in FIG. 2, the various spreader devices are mounted onrespective frames 26 a, 26 b, 26 c, 26 d, and 26 e represented bychain-dotted rectangles. The frames are located alternately above andbelow a common horizontal plane so as to avoid interfering with oneanother.

[0087] The strips 20 a to 20 e coming from different spreader devicesmeet on the roll 25. In order to adjust the positions of the strips sothat they are exactly adjacent, the transverse positions of the framesrelative to the advance direction of the tows, can be adjusted. Thus,each frame, e.g. 26 e, can be moved along transverse guiding slideways28 e under drive from a motor 29 e.

[0088] In a variant, the unidirectional strips can be placed one besideanother in a manner that is not adjacent, but that includes partialoverlap. Smaller tolerance is required compared with placing the stripsexactly edge to edge, however the portions situated along each edge inthe resulting sheet will need to be sacrificed.

[0089] Transverse cohesion can be imparted to the sheet 30 by projectinga liquid compound thereon downstream from the roll 25, said compoundcontaining a chemical bonding agent, e.g. a compound comprising apolymer in solution.

[0090] Various polymers can be used. Advantageously, the polymers usedcan be suitable for being sacrificed, i.e. they should be easy toeliminate, e.g. by being dissolved or by applying heat treatment.Amongst such polymers, mention can be made of polyvinyl alcohol (PVA) orpolyvinylpyrrolydone type polymers that are soluble in water, and ofsoluble polyester. It is also possible to envisage using polymers thatare compatible with a matrix that is deposited at a later stage whenmaking a composite material using a reinforcing fabric made from amultiaxial sheet including the unidirectional sheet. The term “polymercompatible with the matrix” is used herein to designate a polymer, e.g.a resin, having the same kind as or suitable for dissolving in thematrix, or indeed a polymer that is of a different kind but whosepresence in contact with the matrix does not affect the properties ofthe composite material.

[0091] The liquid compound is delivered to nozzles 32 via a feed pipe34. After the compound has been sprayed, the sheet passes between tworolls 36 which are pressed against each other at adjusted pressure so asto distribute the desired quantity of liquid compound uniformly over theentire surface of the sheet 30. Thereafter, the sheet 30 passes beneatha strip dryer 38 for eliminating the solvent contained in the liquidcompound. The coherent sheet 30 can then be stored on a reel 40 that isrotated by a motor 39.

[0092] In a variant, cohesion can be imparted to the sheet by spraying acompound containing a liquid resin, and then curing the resin.Advantageously, a resin is used which can be cured by being exposed toultraviolet radiation, with the strip dryer 38 being replaced by a UVsource. By way of example, such a resin can be a UV-curable acrylate.

[0093] Further techniques could also be used, e.g. dusting a powder ofheat-fusible or thermo-adhesive polymer onto the sheet, or depositing aheat-fusible or thermo-adhesive film or thread on the sheet, and thenexposing it to a heater device. It is also possible to envisage forming“lines of adhesive” on the sheet constituted by an adhesive in solution,with the solvent subsequently being evaporated.

[0094] Depending on the weight and the number of tows used, it ispossible to obtain a sheet 30 of greater or lesser width. Starting fromtows having a relatively large number of filaments, as alreadymentioned, the method has the advantage of enabling wide sheets to beobtained, i.e. sheets that are at least 5 cm wide, that are preferablyat least 10 cm or more wide, while using a limited number of tows, andthus of spreader devices. Another characteristic of the method is toenable thin sheets to be obtained, weighing no more than 300 g/m² and ofuniform thickness.

[0095] A bonding agent can be applied to the sheet for fixing purposesequally well when the sheet is made of continuous filaments and when itis made of discontinuous filaments.

[0096] When the sheet is designed to be used to form fiber reinforcementof a composite material part obtained by densifying the fiberreinforcement with a matrix, it is preferable to select the bondingagent as a function of that use. For example, a bonding agent suitablefor being sacrificed can be used, which is capable of disappearing bybeing dissolved or by the application of heat prior to densification bymeans of the matrix of the composite material. It is also possible touse a bonding agent that is compatible with the matrix, i.e. that iscapable of dissolving in the matrix or of remaining without reactingchemically therewith, so that the properties of the matrix are notdegraded.

[0097] Other methods of fixing that impart sufficient transversecohesion to the sheet to enable it to be handled can also be envisagedwhen the sheet is made up of discontinuous filaments. These relate inparticular to methods of fixing that serve to attach paralleldiscontinuous filaments to one another.

[0098]FIG. 3 shows the sheet 30 formed by the adjacent unidirectionalstrips 20 a to 20 e passing through a device 33 for spraying jets ofwater under pressure onto the sheet while the sheet is passing over ametal plate 33 a. By rebounding on the plate 33 a, the jets of waterperform a moderate amount of matting of the discontinuous filaments.Thereafter, the sheet 30 passes in front of a drying strip 38 prior tobeing stored on the reel 40.

[0099] In another variant shown in FIG. 4, the strip 30 passes through aneedling device 35. This device comprises a needle board 35 a drivenwith vertical reciprocating motion, and a support 35 b over which thestrip 30 passes. The support 35 b has perforations in register with theneedles of the board 35 a. As a result, the needles penetrate throughthe entire thickness of the sheet 30 while displacing the discontinuousfilaments, thereby giving rise to a limited amount of transverse mattingwhich provides the desired transverse cohesion. The needled sheet isstored on the reel 40.

[0100] Although the spreader device shown in FIG. 1 can be used withtows made up of filaments or fibers that are continuous ordiscontinuous, it is most particularly suitable for tows of continuousfilaments.

[0101] Advantageously, the operation of forming a unidirectional sheetor strip made up of discontinuous filaments includes spreading a tow ofcontinuous filaments as shown in FIG. 1, so as to obtain a sheet 20 a ofcontinuous filaments. This is taken to a stretching and bursting device21 (FIG. 5). The stretching and bursting technique is well known per se.It consists in causing the sheet to pass between several successivepairs of drive rolls, e.g. 21 a, 21 b, and 21 c, which are driven atrespective speeds v_(a), v_(b), and v_(c) such that v_(c)>v_(b)>v_(a).By drawing the sheet at increasing speeds, the continuous filaments arebroken. The distance between the pairs of rolls, and in particularbetween 21 a and 21 b determines the bursting pattern, i.e. itdetermines the mean length of the burst filaments.

[0102] After stretching and bursting, the sheet 20′a is stretched,however its weight (per unit area) is significantly reduced comparedwith that of the sheet 20 a. The stretched sheet 20′a made up ofdiscontinuous filaments is optionally juxtaposed side by side with orpartially overlapping other similar sheets 20′b to 20′e, and is thenmade coherent by the above-described moderate matting means, e.g. bybeing subjected to a jet of water under pressure as in theimplementation of FIG. 3, or to needling by a needling device 35, as inthe embodiment of FIG. 4.

[0103] The resulting sheet 30 can be widened so as to further reduce itsweight (per unit area), without the sheet losing its cohesion. Thisability of being widened is given by the cohesion technique used (waterjet or needling).

[0104] Widening can be performed, for example, by causing the coherentsheet 30 to pass over one or more pairs of curved rolls 37 prior tobeing stored on the reel 40.

[0105] It will be observed that the sheet can be widened after it hasbeen stored on the reel 40, e.g. when it is taken from the storage reelin order to form a multiaxial sheet.

[0106] Other known techniques for obtaining unidirectional sheets byspreading tows can also be used, for example the techniques described inRhone Poulenc Fibres documents FR-A-2 581 085 and FR-A-2 581 086. Inthose documents, a tow for spreading is taken to rolls which includeresilient elongate elements at their peripheries that are disposed alonggenerator lines and that are provided with spikes. For the portion ofits path where it is in contact with a roll, the tow is engaged on thespikes and it is spread by the elastic elements extending parallel tothe axis of the roll.

[0107] Making a Multiaxial Sheet

[0108] Reference is now made to FIGS. 6A-6B which show a laying machineconstituting an embodiment of the invention suitable for making acontinuous multiaxial sheet from a plurality of unidirectional sheets,at least one of which can be obtained by a method as described above.

[0109] In the example shown, a multiaxial sheet 50 is made up of threeunidirectional sheets 30 a, 30 b, and 30 c making the following anglesrespectively with the longitudinal direction: 0°, +60°, and −60°. Thesheet at 0° (sheet 30 a), i.e. the “main” sheet, is a coherentunidirectional sheet as obtained by the above-described method, unreeledfrom a reel 40 a. The transverse sheets at +60° (sheet 30 b) and at −60°(sheet 30 c) are unidirectional sheets which can also be coherent sheetsobtained by the above-described method and which are unreeled fromrespective reels 40 b and 40 c. The unidirectional sheets used need notnecessarily have the same width. Thus, in the example, the transversesheets 30 b and 30 c both have the same width which is smaller than thatof the longitudinal sheet 30 a. In general, the transverse sheets willnormally be of a width that is smaller than that of the main sheet (0°).

[0110] It will be observed that the angles formed by the transversesheets relative to the sheet at 0° can be other than +60° and −60°, forexample they can be +45° or −45°, or more generally they can be anglesthat are preferably of opposite sign, but that are not necessarilyequal. It will also be observed that more than two transverse sheets canbe superposed with the 0° sheet, e.g. by adding a sheet at 90° and/or byadding at least one other pair of sheets forming opposite anglesrelative to the longitudinal direction.

[0111] As shown in FIG. 6A, the multiaxial sheet 50 is formed on asupport constituted by a horizontal top segment of an endless belt 42 ofa conveyor 44 passing over a drive roll 46 driven by a motor 47, andover a deflection roll 48 (FIG. 6B). It will be observed that the widthof the belt 42 is narrower than that of the sheet 50 so that the sheetprojects slightly from both sides 42 a and 42 b of the belt 42.

[0112] The sheet is made by fetching juxtaposed segments 30 b at +60°onto the belt 42 and then depositing the sheet 30 a that is oriented at0° thereon, and then bringing over that juxtaposed segments of the sheet30 c oriented at −60°. It is an advantageous feature to be able to makea multiaxial sheet 50 in which the 0° sheet is situated between thetransverse sheets, thereby conferring a symmetrical nature to the sheet50. This is made possible by the cohesion intrinsic to the sheet 30 a.

[0113] Also advantageously, the unidirectional sheet at 0°, as obtainedby a method as described above, is of relatively great width, not lessthan 5 cm, and preferably at least 10 cm, thus making it possible tomake multiaxial sheets of great width.

[0114] The devices 60 for fetching, cutting, and laying successivesegments of the sheets 30 b and 30 c are identical, so only the deviceassociated with the sheet 30 c is described.

[0115] The sheet 30 c is unreeled from the reel 40 c by means of agrasping head 70 having at least one clamp capable of taking hold of thefree end of the sheet 30 c.

[0116] The sheet 30 c is pulled from an edge 42 a of the conveyor belt42 over a length that is sufficient to cover the width of thelongitudinal sheet. The segment thus fetched is cut off in thelongitudinal direction at the edge of the sheet 30 a which is situatedover the edge 42 a of the conveyor belt by means of a cutter device 80.Simultaneously, the cutoff segment of sheet 30 c is fixed by means ofits end which has just been cut so as to conserve its position on theconveyor belt relative to the previously fetched segment, and thusrelative to the sheets 30 a and 30 b which have already been laid.

[0117] In order to cut the sheet 30 c without deformation or fraying,local reinforcement in the form of a segment of film or tape 92 is fixedon each face of the sheet 30 c at each location where it is to be cut.The film 92 can be fixed, for example, by adhesive, by thermo-adhesive,by high frequency welding, by ultrasound welding, . . . by means of adevice 90. For example, a polyethylene film is used that can be fixed bythermo-adhesion. It will be observed that a reinforcing film could befixed over one face only of the sheet 30 c.

[0118] The grasping head 70 is carried by a block 62 which slides in aslideway 64 of a beam 66. By way of example, the block 62 is fixed on anendless cable 68 driven in the slideway 64 by a reversible motor 69. Thebeam 66 supports the reel 40 c, and also the devices 80 and 90 forcutting off and laying segments of the sheet, and for puttingreinforcing film into place.

[0119] A detailed description of how the head 70 and the devices 80 and90 are implemented is given below. It will be observed that the graspinghead can be swivel mounted relative to the block 62 as can the devices80 and 90 relative to the beam 66. As a result, the angle made by thedeposited transverse sheet relative to the longitudinal direction (0°)can easily be modified by appropriately adjusting the orientation of thebeam 66 and by adjusting the positions of the head 60 and of the devices80 and 90 relative to the beam. Operation of the head 70 and of thedevices 80, 90 is controlled by a control unit 100 to which they areconnected by a bundle of cables 102 running along the beam 66.

[0120] A segment of each sheet 30 b and 30 c is fetched, cut off, laid,and fixed while the conveyor 44 is stationary. Thereafter, the conveyoris caused to advance over a length equal to the size of the sheets 30 band 30 c as measured in the longitudinal direction (0°), and the processis repeated. On each advance of the conveyor 44, the same length of thelongitudinal sheet is unreeled.

[0121] After being superposed, the sheets 30 a, 30 b, and 30 c arebonded together. In the example shown in FIG. 6B, this bonding isperformed by needling by means of a needle board 52 which extends acrossthe entire width of the multiaxial sheet 50, as it leaves the conveyor44. During needling, the sheet 50 is supported by a plate 52 a carryinga base felt 52 b, e.g. made of polypropylene, into which the needles canpenetrate without being damaged. Needling is then performed each timethe conveyor advances. Bonding by needling is particularly suitable forsheets made of discontinuous filaments or of continuous filaments thatare not liable to be excessively damaged by the needling.

[0122] A discontinuous web of fibers can be applied to the multiaxialsheet immediately prior to needling, so as to supply discontinuousfibers suitable for being taken by the needles so as to be introducedtransversely into the multiaxial sheet, thereby bonding it.

[0123] After needling, the marginal zones of the multiaxial sheet 50,carrying portions of the reinforcing film 92 can be eliminated by beingcut off by means of rotary cutter wheels 56 situated on both sides ofthe sheet. The resulting multiaxial sheet can be stored on a reel 58driven by a motor 59, synchronously with the intermittent advance of theconveyor 44.

[0124] Reference is now made to FIG. 7 which shows in highlydiagrammatic manner, greater detail of the device 90 for puttingreinforcing films 92 into place by thermo-adhesion.

[0125] Each film 92 is pulled from a respective storage reel 92 a andpasses between two reels 93 a, 93 b, one of which (e.g. 93 a) is coupledto a drive motor (not shown) which may be common to both reels 93 a. Twoclamps 96 are opened and closed under the control of actuators 96 a, andare fixed at the ends of rods 98 secured to the same cylinder of apneumatic actuator 99. The two rods 98 extend respectively above andbelow the path of the sheet 30 c as pulled from the reel 40 c, and theyare of a length that is longer than the width of the sheet.

[0126] Two heating presses 97 are disposed on either side of the path ofthe sheet 30 c. Two blades 94 a co-operating with backing blades 94 bare disposed immediately downstream from the pairs of reels 93 a, 93 bso as to be able to section the films 92 under the control of actuators(not shown).

[0127] A cycle for putting the reinforcing films 92 into place comprisesthe following operations as illustrated in FIGS. 8A to 8C.

[0128] Starting with the rods 98 that carry the clamps 96 in their mostadvanced position, beyond the edge of the sheet 30 c opposite from theedge adjacent to the actuator 99, the films 92 are advanced by means ofthe reels 93 a, 93 b until their free ends are fully engaged in theclamps 96 which are in the open position (FIG. 8A). The drive wheels 93a can be stopped either in response to detecting that the ends of thefilms 92 are home in the clamps 96 by using appropriate sensors, or elseafter the films have been advanced through a predetermined length.

[0129] The clamps 96 are closed under the control of actuators 96 a, thereels 93 a are declutched, and the actuator 99 is controlled to retractthe rods 98 and to pull the films 92 to beyond the edge of the sheet 30c on the same side as the actuator 99 (FIG. 8B).

[0130] The heating presses 97 are applied on either side of the sheet 30c against the segments of film 92 that are situated on each face of saidsheet so as to fix said segments by thermo-adhesion. As soon as thepresses 97 have been applied, the clamps 96 are opened and the blades 94a are actuated so as to cut the films 92, thereby releasing the bladesegments of film during thermo-adhesion (FIG. 8C).

[0131] After the presses 97 have been withdrawn and the sheet 30 c hasbeen advanced, the rods 98 are again brought into the advanced positionby the actuator 99, and the film-laying cycle can then be repeated.

[0132] Reference is now made to FIGS. 9 and 10 which show in greaterdetail but in highly diagrammatic manner the grasping head 70 and thedevice 80 for cutting and fixing segments of the transverse sheet. Thegrasping head 70 comprises a clamp 71 having two elements 71 a and 71 bfor taking hold of the free end of the sheet 30 c. Opening and closingof the clamp 71 are under the control of an actuator 72 which acts onthe top element 71 a. In addition, the clamp 71 is movable between aposition in which it is close to the plane of the conveyor belt 42, anda position in which it is moved away from said plane under the controlof another actuator 73 which is fixed to the block 62 and which supportsthe clamp 71.

[0133] In the vicinity of the edge 42 a of the conveyor belt 42 situatedon the side from which the sheet 30 c is fetched, there is situated aguide device 74 in the form of a clamp. This clamp comprises a topelement 74 a that is movable under the control of an actuator 75 abetween a high position away from the plane of the conveyor belt 42 anda low position that is situated practically in said plane. The clamp 74also has a bottom element 74 b that is movable under the control of anactuator 75 b between a low position situated practically in the planeof the conveyor belt 42 and a high position at a distance from saidplane.

[0134] The cutting device 80 comprises a blade 81 mounted on a support82 situated beneath the plane of the conveyor belt 42. The support 82can slide along the edge 42 a of the belt 42 under the control of anactuator 84. A presser device 85 is disposed above the plane of theconveyor belt 42 so as to press the sheet 30 c onto a support 86 while asegment of the sheet is being cut off. The application of pressure andthe withdrawal of the presser device 85 are controlled by an actuator87. The support 87 and the presser device 85 have respective slots 86 aand 85 a for passing the blade 81.

[0135] The presser device 85 and the support 86 are also heater elementsso as to constitute a heating press capable of clamping against theedges of the multiaxial sheet 50 that is being built up on the side 42 aof the conveyor belt. A heating press made of two similar elements 88under the control of actuators 89 can be provided on the opposite side42 b of the conveyor belt.

[0136] The width of the conveyor belt 42 is less than the width of themultiaxial sheet 50 being built up so as to leave the space required onthe side 42 a for the cutting device 80 and on the side 42 b foroptional heating presses 88.

[0137] A cycle of fetching, cutting off, and fixing a segment oftransverse sheet 30 c comprises the following operations, as illustratedin FIGS. 11A to 11C.

[0138] The free end of the sheet 30 c in the vicinity of the side 42 aof the conveyor belt 42 is held by the clamp 74 with its elements 74 aand 74 b in the high position. The grasping head 70 has its clamp 71 inthe high position and it is situated at the end of its stroke on theside 42 a of the conveyor belt. In this position, the clamp 71 can beclosed by the actuator 72 to take hold of the end of the sheet 30 c(FIG. 11A).

[0139] The clamp 74 is opened by lowering its bottom element 74 b, andthe block 62 is moved by the motor 69 to bring the clamp 71 to the otherend of its stroke, a little beyond the side 42 b of the conveyor belt 42(FIG. 11B).

[0140] The clamp 71 is lowered as is the top element 74 a of the clamp74 so as to press the segment of sheet 30 c against the conveyor belt 42which is already supporting the sheets 30 b and 30 a. The presser device85 is lowered by means of the actuator 87 so as to press the sheet 30 cagainst the support 86. The blade 81 is then moved longitudinally so asto cut the sheet 30 c (FIG. 1C). The sheet 30 c is cut at the locationwhere the reinforcing films 92 have been fixed, with the distancebetween the devices 80, 90 for laying the reinforcing films and forcutting the transverse sheet being equal to the transverse advancedistance of the sheet 30 c, i.e. to the length of the segment of sheet30 c to be cut off.

[0141] The heating elements 85 and 86 are controlled to produce the heatrequired for causing the cutoff portions of the reinforcing films 92 toadhere to the edge of the multiaxial sheet situated on the side 42 a ofthe conveyor belt 42 so as to fix the position of the cutoff segment ofsheet 30 c on this side. The other film portions 92 which remain securedto the free end of the sheet 30 c after cutting can be caused to adhereby means of the heating presses 88 to the other side of the multiaxialsheet 50. As a result, each cutoff segment of the sheet 30 c is held inposition relative to the remainder of the multiaxial sheet duringformation thereof. This avoids any untimely displacement of the segmentsof the transverse sheet during the advances of the conveyor belt 42prior to the multiaxial sheet being finally fixed.

[0142] The clamp 71 can then be opened and returned to its high positionprior to being moved back towards the side 42 a of the conveyor belt,while the clamp 74 is returned to its high position so as to present thefree end of the sheet 30 c in the desired position to the grasping head.

[0143] Variant Embodiments

[0144] The above-described laying machine operates with discontinuousadvance of the multiaxial sheet while it is being formed. In order toincrease production throughput and improve compatibility with theoperation of the means for bonding together the superposedunidirectional sheets when said bonding is performed by sewing or byknitting, it can be preferable to cause the laying machine to operatewith advance that is continuous.

[0145] To this end (FIG. 12), the cutoff segments of transverse sheetare taken hold of by a transfer device 104 to be brought successivelyonto the multiaxial sheet 50 that is being formed and that is advancingcontinuously. The transfer device 104 has two pairs of clamps 104 a, 104b carried by blocks 106 a, 106 b which are movable in translationparallel to the advance direction on either side of the conveyor belt42. To this end, the blocks 106 a and 106 b are fixed on endless cableswhich pass over drive wheels 108 a and 108 b driven by a motor 110 andover two deflector wheels 112 a and 112 b. Two pairs of heating presserwheels 114 a and 114 b serve to fix a segment of transverse sheet bythermo-adhesive of the films 92 at the ends of the segments of sheet, assoon as it has been laid.

[0146] Each segment of transverse sheet is fetched and cut off by across-laying device 60 similar to the machine shown in FIGS. 6A-6B,except that the cutter device 80 is carried by the beam 66 and theheating presses for fixing the cutoff segments of sheet are notprovided.

[0147] Laying is performed by fetching and cutting off each segment bymeans of the cross-laying device and by taking hold of the cutoffsegment, as soon as it has been released by the cross-laying device bymeans of clamps 104 a, 104 b. These are moved synchronously by the motor110 at a determined speed to bring the cutoff segment into contact withthe previously-laid segment and into the desired position (adjacent orwith overlap). Thereafter the clamps 104 a, 104 b are returned to theirinitial position to transfer the following cutoff segment of sheet.

[0148] In another variant, and also for the purpose of increasingproduction throughput, each cross-laying device that fetches, cuts off,and lays successive segments of transverse sheet has a plurality ofgrasping heads that are moved along a path in a closed loop. As aresult, while one grasping head is returning, another grasping head canbe in action.

[0149]FIGS. 13A to 13D show the successive steps of fetching, cuttingoff, and fixing a segment of transverse sheet.

[0150] The cross-laying device differs from that of FIGS. 6A to 11C inthat it has a plurality, e.g. two grasping heads 70 ₁ and 70 ₂ mountedon an endless transporter 76 using a belt or a chain. The transporter 76has its bottom and top lengths extending above the conveyor belt 42,parallel thereto, and in the laying direction for the transverse sheet30 c that is to be laid. The transporter 76 passes over a drive wheel 76a and a return wheel 76 b situated on opposite sides of the conveyorbelt 42. The heads 70 are mounted at opposite locations on thetransporter 76.

[0151] Each head 70 ₁ and 70 ₂ has a shoe 77 fixed at the end of anactuator 78. Connection between a grasping head and the free end of thesheet 30 c is provided by means of adhesive sprayed onto the shoe 77 byan adhesive nozzle 79 situated above the top length of the transporter76 in the vicinity of the end of the return path.

[0152] The cross-laying device of FIGS. 13A to 13D also differs fromthat of FIGS. 6A to 11C in that the presser device 85 is applied andwithdrawn, not under the control of actuator means drivenperpendicularly to the sheet, but by using a pivoting mount. The presserdevice 85 is connected to a support 85 b by means of hinged links 85 c.The hinged links 85 c are driven by a motor member (not shown) to movethe presser device 85 along a circular arc between a front position overthe blade 81, and a rear position in which a passage for the graspinghead is left clear. The support 85 b is movable under drive of anactuator 85 e between a raised position above the plane of the sheet 50and a lowered position substantially level with the sheet 50. It willalso be observed that the guide device 74 of FIGS. 9 to 11C is nowsuperfluous. Operation is as follows.

[0153] Starting with the support 85 b in the high position and thepresser device 85 in the rear position, a grasping head 701 on whichadhesive has been sprayed comes into contact with the free end of thesheet 30 c (FIG. 13A).

[0154] The presser device 85 is raised by means of the links 85 c andthe transporter 76 is driven so that the free end of the sheet 30 c istaken towards the side 42 b of the conveyor belt 42, over the sheet 50(FIG. 13B).

[0155] When the free end of the sheet 30 c has come into position, thetransporter 76 is stopped, the presser device 85 is tilted into itsforward position, thereby holding the sheet 30 c in the tensioned statebetween the grasping head 701 and the presser device 85 (FIG. 13C).

[0156] Thereafter, the actuator 85 e and the actuator 78 of the head 70are controlled to press the sheet 30 c onto the sheet 50 (FIG. 13D). Thesegment is then cut off by means of the blade 81 passing through theslot 85 a. Simultaneously, the edges of the cutoff segment are caused toadhere by means of the presser device 85 and the support 86 constitutinga heating press, and by pressure from the head 70 ₁ on the heatingelement 88. It will be observed that a single heating element 88 isprovided, unlike the embodiment of FIG. 9. At the same time, adhesive issprayed onto the head 70 ₂ by means of the nozzle 79. Thereafter, thehead 70 ₁ is raised and then the transporter 76 is again driven so thata new laying cycle can start using the head 70 ₂ In the above, provisionis made to fix the ends of the transverse sheet segments temporarily bythermo-adhesive along one or both longitudinal edges of the multiaxialsheet, with the marginal portions thereof subsequently being eliminated.

[0157] In a variant, temporary fixing of the transverse sheet segmentscan be provided by means of two longitudinal rows of spikes 49 along theedges 42 a, 42 b of the conveyor belt (FIG. 14). The transverse sheetsegments are engaged at their ends on the spikes 49 when they arepressed against the conveyor belt 42 by lowering the clamps 71, 74 or bymeans of the transfer device of FIG. 12.

[0158] In another variant, the successive segments of the transversesheet can be placed not adjacent to one another, but with partialoverlap (FIG. 15). The degree of overlap is adjusted by adjusting thespeed of the conveyor 44 between two successive transverse sheetsegments being brought into position. Such partial overlap makes itpossible to avoid difficulties that can be encountered when placingtransverse sheet segments edge to edge. Under such circumstances,lightweight transverse sheets are used as can be obtained after beingspread as shown in FIG. 5.

[0159] Although the above-described method of laying transverse sheetsby fetching successive segments constitutes a preferred implementationof the invention, the possibility of using other laying techniques, inparticular when the transverse sheets are of relatively small width, isnot excluded.

[0160] Thus, as shown very diagrammatically in FIG. 16, it is possibleto use a technique of a type similar to that described inabove-mentioned document U.S. Pat. No. 4,677,831. In that technique, theends of the transverse sheets 30 b, 30 c are fixed on cross-layingcarriages 110 which are driven with reciprocating motion in translationparallel to the directions of the transverse sheet. The sheets 30 b and30 c are unreeled from reels (not shown) optionally carried by thecross-laying carriages. At each end of the stroke of a cross-layingcarriage, the transverse sheet is turned by passing over spikes 111carried by the conveyor belt 42 along each of its longitudinal sides.

[0161]FIG. 6B shows superposed sheets being bonded together by needling.Other bonding methods can be used.

[0162] Thus, FIG. 17 shows bonding by stitching by means of a device 120situated immediately downstream from the conveyor 44. The stitching canbe performed using various different stitches, e.g. chain stitch 122, asis conventional. By way of example, the sewing thread 124 used can be athread of polyester, glass, carbon, aramid, . . . . It is also possibleto provide bonding by knitting, e.g. using a zigzag knitting stitch.

[0163]FIG. 18 shows bonding by means of heat-fusible threads which areintroduced between the unidirectional sheets. A first heat-fusiblethread 130 is placed on the sheet segments 30 b by a cross-laying device131 prior to the sheet 30 a being laid, and a second heat-fusible thread132 is placed on the sheet 30 a by a cross-laying device 133 prior tothe sheet segments 30 c being laid. Immediately downstream from theconveyor 44, the multiaxial sheet 50 passes between two heater rolls 124that cause the threads 130 and 132 to melt, thereby providing cohesionfor the multiaxial sheet. By way of example, the threads 130 and 132 areglass threads coated in polypropylene. Instead of heat-fusible threads,it would be possible to use a heat-fusible film, or a thermo-adhesivefilm or thread.

[0164] Finally, FIG. 19 shows bonding by adhesive. Strips 140 and 142for spraying adhesive agent are disposed across the conveyor belt 42immediately downstream from the station for laying the unidirectionalsheet 30 a and the station for laying the unidirectional sheet 30 c.Immediately downstream from the conveyor 44, the multiaxial sheet 50passes between two rolls 144.

[0165] When cohesion of the unidirectional sheets is obtained by aheat-fusible or thermo-adhesive bonding agent, bonding between theunidirectional sheets can also be obtained by thermally reactivating thebonding agent.

[0166] The method and the machine for laying as described above serve tomake multiaxial sheets comprising an arbitrary number of superposedsheets. Thus, it is possible to form a multiaxial sheet that does nothave a longitudinal unidirectional sheet (0°) by placing at least twotransverse unidirectional sheets. In this case, and preferably, thetransverse sheets comprise at least one pair of sheets whose directionsare at opposite angles relative to the longitudinal direction,optionally together with a transverse sheet at 90°. When a longitudinalunidirectional sheet is provided, as already mentioned, at least onepair of transverse sheets are placed on opposite faces of thelongitudinal sheet and at opposite angles relative thereto; in this casealso it is possible to add at least one transverse sheet at 90°.

[0167] The resulting multiaxial sheets can be used for making thereinforcement of composite material parts, e.g. by well-known techniquesof draping or needling superposed plies. The resulting reinforcement isthen densified by a matrix obtained by chemical vapor infiltration or bya liquid process (impregnating with a matrix precursor in the liquidstate, e.g. resin, followed by transforming the precursor, e.g. by heattreatment), or indeed by califaction. With califaction, the preform isimmersed in a liquid precursors of the matrix and the preform is heated,e.g. by contact with an inductor core or by direct coupling with aninductor coil, such that the precursor is vaporized on coming intocontact with the preform and can infiltrate to form the matrix by beingdeposited within the pores of the preform.

EXAMPLES

[0168] Examples of making multiaxial sheets are described below by wayof illustration.

Example 1

[0169] A tow of high-strength carbon fibers constituted by 480,000continuous filaments (480 K) weighing 30,000 tex, having breakingstrength in traction of 3600 MPa and a modulus of 250 GPa was spreadover a width of 150 mm by means of an installation similar to that ofFIG. 1. The spread tow was subjected to a stretching and burstingoperation during which the continuous filaments were transformed intodiscontinuous filaments, the majority of which were of a length lying inthe range 25 mm to 170 mm. During bursting, the spread tow was subjectedto stretching by a factor of 2 and its weight (per unit area) wasreduced, giving a unidirectional sheet having a width of 150 mm and aweight of 110 g/m².

[0170] The sheet was fixed by disorienting the fibers slightly, thegreat majority of them remaining parallel to the sheet direction. Thedisorientation was performed by subjecting the sheet situated over ametal plate to a jet of water under a pressure of at least 100 bars.

[0171] The resulting sheet was quite capable of being handled.

[0172] Two similar unidirectional sheets were laid by means of a machinesimilar to that shown in FIGS. 6A and 6B to form angles of +45° and −45°relative to the longitudinal direction (0°) of the resulting sheet. Thesheets were bonded together by light needling, the density of needlingbeing about 20 strokes/cm². A sheet was obtained having two axes +45°and a weight of 220 g/m².

[0173] Plies were cut out from the two-axis sheet and superposed so asto make reinforcement for a carbon-carbon composite material part to bemanufactured. The plies were bonded together by needling while they werebeing superposed, in well-known manner, e.g. as disclosed in documentU.S. Pat. No. 4,790,052.

[0174] The resulting preform was densified by a carbon matrix depositedby chemical vapor infiltration.

Example 2

[0175] The two-axis sheet of Example 1 was fixed not by needling, but bystitching using a zigzag knit stitch parallel to the longitudinaldirection. The knitting thread was a 150 dtex cotton thread having twostrands. A two-axis sheet was obtained that was quite capable of beinghandled.

Example 3

[0176] The tow of Example 1 as spread and fixed by a jet of water afterstretching and bursting was enlarged by being passed over curved bars toincrease its width from 80 mm to 120 mm. Two similar unidirectionalsheets obtained in this way were laid at +45° and −45°, as in Example 1,but with 50% overlap between successively-laid segments of sheet. Thetwo-axis sheet was fixed by needling, as in Example 1. A two-axis sheetwas obtained weighing 530 g/m² and that was quite capable of beinghandled.

Example 4

[0177] Four tows of 320 K filaments each and constituted bydiscontinuous carbon fibers were spread side by side to form aunidirectional sheet that was 600 mm wide and weighed about 140 g/m².The sheet was fixed by pre-needling at a density of 30 strokes per cm².

[0178] Three similar unidirectional sheets were laid in directionsrespectively equal to 0°, +60°, and −60°, by means of a laying machineas illustrated in FIGS. 6A-6B. The sheets were bonded together byneedling, using a density of 30 strokes per cm². The resultingthree-axis sheet weighed 420 g/m². It was particularly suitable formaking preforms for composite material parts by stacking and needlingflat plies, or by winding and needling on a mandrel.

Example 5

[0179] Four high-strength carbon tows each having 50 K filaments andmade of preoxidized polyacrylonitrile (PAN) carbon precursor were spreadand burst together as described in Example 1. The resultingunidirectional strip was 8 cm wide and weighed 170 g/M².

[0180] A carbon fiber tow having 320 K filaments, made of isotropicpitch precursor, was burst in the same manner so as to obtain aunidirectional strip having a width of 8 cm and a weight of 230 g/m².

[0181] Two burst strips of that type based on isotropic pitch precursorwere interleaved with eight burst strips of the preceding type based onpreoxidized PAN precursor and the assembly was passed once through agill box or an “intersecting” type machine in which all ten strips werecombed and stretched so as to obtain a burst sheet made up of anintimate mixture of different precursor fibers, weighing 250 g/m², and awidth of 10 cm.

[0182] The resulting hybrid sheet was fixed by being subjected to a jetof water under pressure, with the sheet then being situated over a metalplate.

[0183] A three-axis sheet with axes at 0°, +60°, and −60° was made usingthree unidirectional sheets as made in that way.

Example 6

[0184] Tows of high-strength carbon fibers and each having 12 Kfilaments were spread so as to bring their width to about 7 mm. Threeunidirectional sheets of width equal to about 100 mm and weighing 125g/m² were formed by juxtaposing spread tows, as was a unidirectionalsheet of width equal to 100 cm and having the same weight (per unitarea). The sheets were fixed by spraying a bonding agent in liquid formas shown in FIG. 1. The bonding agent used was a water soluble polyvinylalcohol (PVA). The quantity of PVA used was 2.1% by weight relative tothe weight of the sheets.

[0185] A multiaxial sheet was made using a machine of the type shown inFIGS. 6A and 16, by using a 100 cm wide unidirectional sheet as thelongitudinal sheet (0°) together with 100 mm wide unidirectional sheetsas the transverse sheets which were laid in the following directions:90°, +45°, and −45°, the sequence being 90°/+45°/0°/−45°. The foursheets were bonded together by stitching using a continuous 76 dtexpolyester thread. A 6 gauge was used and a chain stitch type stitchhaving a pitch of 4 mm was employed.

[0186] After the unidirectional sheets had been bonded together, themultiaxial sheet was de-oiled to eliminate the PVA and to make itcompatible with the intended utilizations.

[0187] Such a multiaxial sheet is suitable, for example, for beingimpregnated with an epoxy resin to make composite material parts.

Example 7

[0188] A +45°/0°/−45° “mirror” multiaxial sheet was made from alongitudinal unidirectional sheet (0°) of high modulus M46JB type carbonfibers from the French company SOFICAR and from two transverseunidirectional sheets (+45°, −45°) of high strength T700SC type carbonfibers from the Japanese company TORAY.

[0189] The 0° sheet was formed by spreading 12 K-filament tows to awidth of 3 mm and by juxtaposing the spread tows to obtain a 300 mm widesheet weighing 150 g/m².

[0190] The +45° and −45° sheets were formed by spreading 12 K-filamenttows to a width of 8 mm and by juxtaposing the spread cables so as toobtain 130 mm wide sheets weighing 100 g/m².

[0191] The unidirectional sheets were fixed by immersion in a bathcontaining an epoxy resin emulsion. The sheets were passed betweenpresser rolls to wring out the resin so that its final concentration was1.8% by weight relative to the weight of the sheet.

[0192] Laying was performed in the +45°/0°/−45° sequence, with thetransverse sheet segments being juxtaposed edge to edge.

[0193] Bonding between the unidirectional sheets was provided by placinga heat-fusible copolyamide thread between the sheet every 100 mm, and bycausing the multiaxial sheet to pass between two heater rolls, as shownin FIG. 17.

[0194] After being impregnated with an epoxy resin that is chemicallycompatible with the bonding agent used for imparting cohesion to theunidirectional sheets, the resulting multiaxial sheet was used to makecarbon/epoxy composite masts for boats.

Example 8

[0195] A 90°/+30°/−30° three-axis sheet was made from three identicalunidirectional sheets. Each unidirectional sheet was made by spreading50 K-filament tows of high-strength carbon fibers to a width of 18 mmand by juxtaposing the spread tows to obtain a 200 mm wide sheetweighing 200 g/m². The unidirectional sheet was fixed by spraying anemulsion of vinylpyrrolydone polymer at a concentration corresponding to0.8% dry weight.

[0196] The unidirectional sheets were superposed in segments that werejuxtaposed edge to edge and bonded together by stitching using a 76 dtexpolyester thread using a chain stitch type sewing stitch. A 6 gauge wasused with a sewing pitch of 4 mm.

[0197] The resulting multiaxial sheet could then be de-oiled toeliminate the bonding agent used to impart cohesion to theunidirectional sheets.

Example 9

[0198] A 0°/+45°/90°/−45° multiaxial sheet was made from four identicalunidirectional sheets. Each unidirectional sheet was formed by spreadingglass fiber threads of the “Roving 2400 tex” type. The spread threadswere juxtaposed longitudinally and held parallel to one another by aheat-fusible thread placed transversely about every 5 cm, such that thecohesive unidirectional sheet formed in this way weighed 300 g/m² and awidth of 20 cm.

[0199] Using these unidirectional sheets, the multiaxial0°/+45°/90°/−45° sheet was formed with the +45°, 90°, and −45° sheetsbeing constituted by segments juxtaposed edge to edge. The fourunidirectional sheets were bonded together by lines of stitching using apolyester thread. The stitch had a length of about 10 mm and the linesof stitching were spaced apart by about 40 mm.

[0200] A cohesive glass fiber multiaxial sheet was obtained havingstitching at very low density, such that the multiaxial sheet retainedsufficient flexibility to be easily pre-formed, and it presented asmooth surface state.

Example 10

[0201] A four-axis sheet was made from four identical unidirectionalsheets. Each unidirectional sheet was formed by spreading 12 K-filamentcarbon threads supplied by the Japanese company “Toray” under thereference “T700SC”. The spread threads were juxtaposed and held togetherby a heat-fusible thread placed transversely about every 5 cm, such thatthe resulting cohesive unidirectional sheet weighed 150 g/m² and a widthof 10 cm.

[0202] From those unidirectional sheets, two multiaxial sheets of typesA and B were formed as follows:

[0203] A: −45°/0°/+45/90°

[0204] B: +45°/0°/−45°/90°.

[0205] The unidirectional sheets forming those two multiaxial sheetswere held together by stitching with a polyester thread. Low densitystitching was performed with a stitch that was 10 mm long and with linesof stitching that were spaced apart by 25 mm.

[0206] Multiaxial A and B sheets can be superposed so as to build up a“mirror” stack having the same number of sheets disposed on either sideof a middle longitudinal plane of symmetry, with each A sheet or B sheetbeing symmetrical to a B sheet or an A sheet about said plane.

[0207] For example, one “mirror” stack had the following succession ofsheets: A/A/A/B/B/B. That stack was made cohesive by stitching in itsthickness with an aramid thread, e.g. a 217 dtex “Kevlar” (registeredtrademark) thread with stitching being performed at a pitch of 5 mm×5mm.

1. A fibrous unidirectional sheet showing transverse cohesion andconstituted by juxtaposed unidirectional strips obtained from spreadedtows having at least 12 K filaments per tow, the sheet weighing not morethan 300 g/m² and being of width of not less than 5 cm.
 2. A sheetaccording to claim 1, characterized in that it is made of fibers of amaterial selected from carbon, ceramics, carbon or ceramic precursors,glasses, and aramids.
 3. A sheet according to claim 1, characterized inthat it is made of continuous filaments.
 4. A sheet according to claim1, characterized in that it is made of discontinuous filaments.
 5. Asheet according to claim 3, characterized in that cohesion is impartedthereto by the presence of a bonding agent.
 6. A sheet according toclaim 5, characterized in that the bonding agent is suitable for beingeliminated.
 7. A sheet according to claim 6, characterized in that thebonding agent is a water-soluble polymer.
 8. A sheet according to claim4, characterized in that cohesion is imparted thereto by lightly mattingdiscontinuous filaments.
 9. A sheet according to claim 4, characterizedin that cohesion is imparted thereto by needling.
 10. A method ofmanufacturing a composite material part comprising comparing fiberreinforcement from at least one unidirectional sheet according to claim5 and densifying the fiber reinforcement by means of a matrix, themethod being characterized in that a unidirectional sheet is used thathas had cohesion imparted thereto by the presence of a bonding agentthat is compatible with the matrix.
 11. A composite material partcomprising fiber reinforcement densified by a matrix, the part beingcharacterized in that the fiber reinforcement comprises at least oneunidirectional sheet according to claim
 1. 12. A multiaxial fiber sheetin the form of a continuous strip having a longitudinal direction,comprising a plurality of superposed unidirectional sheets of differencedirections that are bonded together, the sheet being characterized inthat it includes at least one unidirectional sheet according to claim 2.13. A multiaxial sheet according to claim 12, characterized in that itis constituted by two unidirectional sheets at angles of +45° and −45°to the longitudinal direction of the multiaxial sheet.
 14. A multiaxialsheet according to claim 12, characterized in that it comprises a mainunidirectional sheet oriented in the longitudinal direction of themultiaxial sheet and at least two transverse longitudinal sheets eachdisposed on a respective face of the main sheet and extending indirections that make opposite angles with the direction of the mainsheet.
 15. A composite material part comprising fiber reinforcementdensified by a matrix, the part being characterized in that the fiberreinforcement comprises at least one multiaxial sheet according to claim12.
 16. A sheet according to claim 2, characterized in that: it is madeof continuous or discontinuous filaments; cohesion is imparted theretoby the presence of a bonding agent; the bonding agent is suitable forbeing eliminated and is a water-soluble polymer.
 17. A compositematerial part comprising fiber reinforcement densified by a matrix, thepart being characterized in that the fiber reinforcement comprises atleast one unidirectional sheet according to claim
 8. 18. A compositematerial part comprising fiber reinforcement densified by a matrix, thepart being characterized in that the fiber reinforcement comprises atleast one unidirectional sheet according to claim
 9. 19. A multiaxialfiber sheet in the form of a continuous strip having a longitudinaldirection, comprising a plurality of superposed unidirectional sheets ofdifferent directions that are bonded together, the sheet beingcharacterized in that it includes at least one unidirectional sheetaccording to claim
 8. 20. A multiaxial fiber sheet in the form of acontinuous strip having a longitudinal direction, comprising a pluralityof superposed unidirectional sheets of different directions that arebonded together, the sheet being characterized in that it includes atleast one unidirectional sheet according to claim
 9. 21. A multiaxialsheet according to claim 19, characterized in that it is constituted bytwo unidirectional sheets at angles of +45° and −45° to the longitudinaldirection of the multiaxial sheet.
 22. A multiaxial sheet according toclaim 20, characterized in that it is constituted by two unidirectionalsheets at angles of +45° and −45° to the longitudinal direction of themultiaxial sheet.
 23. A multiaxial sheet according to claim 19,characterized in that it comprises a main unidirectional sheet orientedin the longitudinal direction of the multiaxial sheet and at least twotransverse longitudinal sheets each disposed on a respective face of themain sheet and extending in directions that make opposite angles withthe direction of the main sheet.
 24. A multiaxial sheet according toclaim 20, characterized in that it comprises a main unidirectional sheetoriented in the longitudinal direction of the multiaxial sheet and atleast two transverse longitudinal sheets each disposed on a respectiveface of the main sheet and extending in directions that make oppositeangles with the direction of the main sheet.